Brief overview of invention: In 1930 the famous physiologist Otto Warburg reported that one of the most universal characteristics of tumors is their acidity. Subsequent studies have confirmed that much of the interstitial space of tumors is acidic, ranging from about pH 6.0 to about pH 7.2. James Summerton, the present inventor, has devised novel compositions called “onco-tools” which are designed to exploit this acidity to afford early detection and safe and effective treatment of a broad range of tumors. Key to the mechanism of action of onco-tools are components called “pH-switches”. At the pH of normal tissues (pH 7.4) a pH-switch exists in an anionic hydrophilic form which has a very low affinity for tissues, but when the pH-switch enters an acidic area in a tumor it undergoes a pH-mediated switch to a non-ionic lipophilic form that has a high affinity for tissues. As a consequence, an onco-tool does not bind to normal tissues, but it is sequestered in acidic areas of tumors. Onco-tools which are not sequestered in a tumor will be excreted by the kidneys. Onco-tools also contain a cargo component that serves to report the presence of the tumor (diagnostic use) or serves to kill the tumor (therapeutic use). Initial compositions and methods which exploit tumor acidity for detecting and treating tumors are described in two patent applications previously submitted by the present inventor (one of these applications is pending and one is allowed but not yet published or issued). The present patent application by the same inventor describes subsequent improvements and extensions in the design, preparation, properties, applications, and methods of use of these novel onco-tools.
Acidity in tumors: It has been known for many decades that most or all tumors larger than about 1 millimeter in diameter contain hypoxic/acidic areas. The likely cause of these hypoxic/acidic areas is that for tumors to grow larger than about 1 mm in diameter they must induce new blood vessels, and such tumor-induced blood vessels, particularly the capillaries, are abnormal, being too widely spaced, torturous in path, and their walls are excessively permeable. As a consequence, cells more than a few tens of microns from such tumor capillaries commonly are chronically hypoxic and the interstitial space surrounding them is acidic, ranging from as low as about pH 6.0 in areas most distant from capillaries, up to about pH 7.2 closer to capillaries, with pH values in the range of about 6.5 to 6.8 being most common. This acidity is probably due in substantial part to the hypoxia causing the tumor cells to shift to glycolytic metabolism, which leads to their producing and excreting lactic acid. While tumor cells at near-normal pH in close proximity to capillaries have high metabolic rates and fast cell division, those tumor cells in hypoxic/acidic areas a greater distance from capillaries have low metabolic rates and divide slowly or not at all. These slow and non-dividing tumor cells are called quiescent.
The quiescent tumor cells in hypoxic/acidic areas of tumors probably constitute the greatest impediment to long-term success with conventional cancer therapies. This is because while conventional cancer therapies (radiation and chemotherapeutics) are fairly effective in killing the more vulnerable fast-dividing cells, such as those tumor cells in close proximity to tumor capillaries, most such therapies have been explicitly selected for their ability to spare slow-dividing and non-dividing cells typical of most normal tissues. Therefore, not surprisingly, conventional cancer therapies are also rather ineffective against slow-dividing and non-dividing quiescent tumor cells. As a consequence, tumor treatments typically kill predominantly the fast-dividing cells of a tumor while sparing quiescent cells of that tumor. This initial killing of the more vulnerable fast-dividing tumor cells causes the tumor to go into remission, but after those killed cells have been disposed of by the body's normal cleanup processes, all too often the treatment-resistant quiescent cells in the hypoxic/acidic areas of the tumor slowly regain access to adequate oxygen, nutrients, and waste disposal—allowing them to revert to high metabolic rate and fast cell division. This rejuvenation of the previously quiescent tumor cells commonly leads to the dreaded relapse that kills so many patients.
Prior efforts to exploit acidity in tumors: The hypoxic/acidic properties of tumors have been known for over 75 years and it has long been speculated that such properties might be exploitable for therapy. However, to date it appears the most successful efforts to exploit these properties have focused on the hypoxia. Specifically, substances have been developed which exhibit minimal cytotoxicity in normoxic cells, while exhibiting considerable cytotoxicity in hypoxic cells. One such agent has progressed to the clinical trials stage.
Compared to exploiting the hypoxia in tumors, until quite recently there appears to have been much less success in exploiting the acidity of tumors. One unsuccessful approach was based on the observation that acid pH in tissues acts to sensitize those tissues to thermal damage. However, efforts to exploit this acid-mediated sensitivity of cells gave disappointing results.
Another approach relating to acidity in tumors is based on the fact that the low pH in tumors ionizes weak-base cytotoxic agents and thereby renders them membrane-impermeable, which in turn results in preferential reduction of entry of a number of such weak-base agents into cells in acidic areas of tumors relative to entry of such agents into cells in areas of more normal pH. In this regard, rather than attempting to exploit the low pH in the tumor, efforts were instead focused on raising the pH in the tumors as a means to partially de-ionize and thereby enhance the entry of such weak-base cytotoxic agents into cells of the tumor, and such efforts have met with some success.
Related art: Still another approach, and one which relates somewhat to the present invention, relies on the fact that the low pH in the interstitial space of tumors will effect partial de-ionization of weak-acid cytotoxic agents. As a consequence of this lesser degree of ionization of weak-acid agents in acidic areas of tumors, one would expect that such agents should show enhanced cell entry and hence greater cytotoxicity against cells in the acidic areas of tumors. This expectation has been tested by Kozin et al., wherein they took measures to make tumors in tumor-bearing mice more acidic by established methods (Cancer Research Vol. 61, pages 4740-4743(2001)). They reported that, as predicted, a reduction of about 0.3 pH units in the tumors coincided with a modest 1.7-fold improvement in tumor growth delay afforded by the weak-acid (pKa 5.8) cytotoxic agent, Chlorambucil (shown in FIG. 1). In the conclusion to their paper they wrote: “To our knowledge, CHL (Chlorambucil) is the only clinical therapeutic that is a weak-acid with the appropriate pKa≦6.5. This study thus provides a rationale for the design of novel, potent drugs exhibiting similar weak-acid properties and for which diffusion contributes to intracellular uptake. As also shown here, the combined use of such compounds with radiation and/or modulators of the pH gradient provide additional opportunities for maximizing the therapeutic response.” In this paper, the authors state that their results with the off-the-shelf chemotherapeutic, Chlorambucil, provide a rationale for designing new weak-acid anti-cancer drugs which may show high efficacy—but no guidance is given for design criteria or prospective molecular structures for such drugs, nor is any guidance given on what specific properties are desirable, nor is any guidance given on how to go about designing and preparing such weak-acid drugs, nor is any guidance given concerning applications or methods of use of such drugs.
Other art which relates more closely to the current invention is the following. In 1993 James Summerton, the present inventor, began development of novel transporter peptides designed from first principles to exploit the pH differential between acidified endosomes (pH 5.0) and the cytosol of cells (pH 7.4) for the purpose of transporting non-ionic antisense oligos across the endosomal membrane into the cytosol of cells (Summerton & Weller, U.S. Pat. No. 6,030,941). Those transporter peptides were designed to convert to a non-ionic/moderately-lipophilic form at the pH within late-stage endosomes (about pH 5.0), and then pass through the endosomal membrane. As they entered into the neutral pH of the cytosol, the transporter peptides were designed to revert to their anionic/hydrophilic form, with that re-ionization and solvation providing the motive force for pulling the attached antisense oligo across the endosomal membrane. While those early transporter peptides resemble in several aspects some of the onco-tools of the current invention, it should be noted that: a) those early transporter peptides are ineffective for discriminating between normal tissues and acidic areas of tumors; and, b) the claims in the 941 patent covering those early transporter peptides do not read on the onco-tools of the current invention; and, c) the claims in the current patent application do not read on those early transporter peptides disclosed in the 941 patent.
Additional art that is still more closely related to the present invention, which was also developed by the same inventor and which has not yet been disclosed to the public, is the following. Several years ago Summerton altered the transporter peptide design in order to produce specialized peptides explicitly designed to exploit the small pH differential between normal tissues and acidic regions of tumors. This was for the purpose of developing compositions which would be selectively sequestered in acidic areas of tumors. The resultant peptides were further adapted to both report the presence of tumors (for diagnostic use) and to kill the quiescent cells in acidic areas of tumors (for therapeutic use). These peptide compositions (now called “onco-tools”) and their use for detecting and treating tumors are disclosed in two US patent applications submitted by the present inventor—one application is pending (Ser. No. 11/069,849) and one has been allowed but has not yet issued (Ser. No. 11/069,387). Neither has yet been published or otherwise disclosed to the public.